Section 2.3—Chemical Formulas We need to be able to read the formulas for chemicals in the antacids! Objective: Explain and use nomenclature rules of writing ionic and covalent chemical formulas

Binary Ionic compounds

Recall these Definitions: Binary Ionic Compound- compound containing two elements—one metal and one non-metal – bonded through an ionic bond. + Cation + Cation - Anion - Anion Ionic Compound Ionic bond- bond formed by attraction between + and - ions

Ionic Charges Think about it: If ionic bonds are the attractive force between a cation (+ charge) and an anion (- charge) AND that an ionic compound contains a metal and a nonmetal, it must also be true that the metals must be charged and the nonmetals must be charged! HOW CAN WE DETERMINE THAT CHARGE??

Charges for these elements will be GIVEN by a roman numeral following the cation’s name ex. iron (III) oxide: Fe is +3 Variable Positive Charges

Writing Formulas for Binary Ionic Compounds To write these formulas:  Write the symbol & charge of the first element (the metal, cation)  Write the symbol & charge of the second element (the non-metal, anion)  Add more of the cations and/or anions in order to have a neutral compound  Use subscripts to show how many of each type of ion is there.

Example #1 Sodium chloride

Example #1 Sodium chloride Cation Anion Na +1 Cl -1 NaCl

Example #1 Sodium chloride Cation Anion Na +1 Cl -1 NaCl Na +1 Cl = 0 The compound is neutral…no subscripts are needed.

Example #2 Calcium bromide

Example #2 Calcium bromide Cation Anion Ca +2 Br -1

Example #2 Calcium bromide Cation Anion Ca +2 Br -1 CaBr 2 Ca +2 Br = +1 Ca +2 Br -1 Br -1 The subscript “2” is used to show that 2 anions are needed = 0

Let’s Practice Example: Write the following chemical formulas Cesium chloride Potassium oxide Aluminum sulfide Calcium bromide

Let’s Practice CsCl K2OK2O Al 2 S 3 CaBr 2 Example: Write the following chemical formulas Cesium chloride Potassium oxide Aluminum sulfide Calcium bromide

Shortcut Criss-cross the NUMBER of the charge to obtain the subscripts. Reduce if necessary. Example1: Al +3 S -2  Al 2 S 3 Example 2: Ca +2 O -2  Ca 2 O 2  CaO

Polyatomic Ionic Compounds

Recall these Definitions Polyatomic Ion- a group of atoms that are bonded together and have an overall charge + Cation + Cation Polyatomic Ionic Compound- compound containing at least one polyatomic ion - Polyatomic Anion Polyatomic Ionic Compound

To write these formulas:  Write the symbol & charge of the cation & anion; if either is a polyatomic ion, look up its charge.  Add additional cations or anions in order to have a neutral compound (OR use the criss- cross shortcut.)  Use subscripts to show the number of ions When using subscripts with a polyatomic ion, you MUST put the polyatomic ion in parenthesis. Identifying & Naming Polyatomic Ionic

Example #3 Sodium carbonate

Example #3 Sodium carbonate Cation Polyatomic Anion Na +1 CO 3 -2

Example #3 Sodium carbonate Cation Polyatomic Anion Na +1 CO 3 -2 Na 2 CO 3 Na + CO = -1 Na + Na + CO 3 2- The subscript “2” is used to show that 2 cations are needed = 0

Example #4 Magnesium nitrate

Example #4 Magnesium nitrate Cation Polyatomic Anion Mg +2 NO 3 -1

Example #4 Magnesium nitrate Cation Polyatomic Anion Mg +2 NO 3 -1 Mg(NO 3 ) 2 Parenthesis are used to show 2 anion groups are needed. Mg +2 NO = +1 Mg +2 NO 3 - NO 3 - The subscript “2” is used to show that 2 anions are needed = 0

Let’s Practice Example: Write the following chemical formulas Sodium nitrate Calcium chlorate Aluminum sulfite Calcium hydroxide Ammonium Phosphate

Let’s Practice NaNO 3 Ca(ClO 3 ) 2 Al 2 (SO 3 ) 3 Ca(OH) 2 (NH 4 ) 3 PO 4 Example: Write the following chemical formulas Sodium nitrate Calcium chlorate Aluminum sulfite Calcium hydroxide Ammonium Phosphate

Binary Covalent Compounds

Definition Binary Covalent Compound compound made from two non-metals that share electrons Non metal Non metal Non metal Non metal Covalent compound Covalent bond atoms share electrons

How we named them:  Prefixes were used to indicate the number of atoms of each element  Example: N 5 O pentanitrogen monoxide Identifying & Naming Binary Covalent

Writing Formulas To write these formulas:  Write the symbols of the first and second element  Translate the covalent prefixes (assume the first element is “1” if there’s no prefix) into subscripts to show number of atoms. Atoms DO NOT form charges when bonding covalently…you DO NOT need to worry about charges with this type!

Example #7 Dinitrogen Tetraoxide

Example #7 Dinitrogen Tetraoxide N O “Di-” = 2 “Tetra-” = 4 N2O4N2O4

Example #8 Silicon dioxide

Example #8 Silicon dioxide Si O “Mono-” is not written for the first element “Di-” = 2 SiO 2

Let’s Practice Example: Write the following chemical formulas Carbon monoxide Nitrogen dioxide Diphosphorus pentaoxide

Let’s Practice CO NO 2 P 2 O 5 Example: Write the following chemical formulas Carbon monoxide Nitrogen dioxide Diphosphorus pentaoxide

Diatomic Elements Some elements are so chemically reactive that they cannot and do not exist in nature as single atoms. If they do not find an atom of another element to bond to, they will bond to an atom of their same kind. The formula for these such elements ALWAYS has a “2” subscript. These elements are: hydrogen, H 2 chlorine, Cl 2 nitrogen, N 2 bromine, Br 2 oxygen, O 2 iodine, I 2 fluorine, F 2

Common Names & Formulas There are a few compounds that are known by a common name, rather than a scientific one. H 2 O is known as water. NH 3 is known as ammonia. (Note: do not confuse this with the ion ammonium, NH 4 + )